It is known to use solid-state imagers for imaging x-rays. It is desirable to synchronize image capture by the image sensor with output of an x-ray generator. This may be done by triggering the sensor on when x-rays are being received at the sensor and triggering the sensor off when the x-rays are no longer being received. This may enable the sensor to be cleared of signal accumulated from dark current before imaging. Since the image capture is closely synchronized to the source, with little or no loss of image signal, the radiation or x-ray dose (e.g. to a patient) is minimized.
In a known arrangement the image sensor is electrically connected to an x-ray source and image acquisition is controlled by a timer of the x-ray source. However, this requires customizing the sensor to the x-ray source.
U.S. Pat. No. 5,331,166 discloses an x-ray detecting device including an x-ray intensity detecting element, such as a phototransistor, in a casing adjacent to an x-ray fluorescent element of the detecting device. When an accumulated x-ray dose becomes equal to a preset x-ray dose required to obtain optimum image quality, x-ray radiation is stopped.
U.S. Pat. No. 5,510,623 discloses a CCD image sensor array which has a two-block, full-frame, parallel register structure which feeds into a single central serial read-out register. If the centre read-out register is not light-shielded, the register can be used to detect the onset and termination of an exposure. An external threshold circuit is used to determine a beginning and end of incoming radiation. Prior to illumination a background level, including dark signal generation, is monitored. An integration period is begun when the threshold circuit detects an increase in signal. As illumination ends, the same threshold detection methodology is used to detect the end of illumination.
In U.S. Pat. No. 5,694,448 a CCD solid-state imager is continually clocked out in a wait period prior to irradiation by an x-ray source. A signal derived from the CCD imager is compared with a predetermined threshold to determine onset of irradiation by the x-ray source, which appears as a rapid change in the signal level. The predetermined threshold may be selected dependent on ambient conditions to allow for consequential changes in dark current.
U.S. Pat. No. 5,912,942 discloses that in preference to an Active Pixel Sensor (APS) CMOS away incorporating several event trigger diodes which are monitored by computer to determine a start and end of an x-ray exposure, since event trigger diodes lower the manufacturing yield of the array, and in any case the event trigger diodes may be obscured by the object being imaged, preferably, instead, data stored in the entire array is constantly read out. Once readout of the entire array is completed, the data is stored in a memory as a frame and compared to a dark current frame before exposure to x-rays. If some of the data in the frame is sufficiently different, it is determined that the x-ray source was turned on during the frame and the frame is stored. The subsequent fame is also stored and the frame after that stored as a dark frame, in which it is known that no x-radiation occurred, since the frame time is set to be greater than the exposure time. The data in the dark frame is subtracted from the first and second frames and the first and second frames added to obtain the exposure data.
U.S. Pat. No. 6,002,742 discloses a number of detector points for x-radiation provided at or in a CCD cell to minimize an accumulated radiation dose to what is needed for exact exposure of an image. Radiation is detected by at least three sensing diodes behind an image array. Alternatively, the CCD image array is used as the x-ray sensor.
In U.S. Pat. Nos. 6,307,915 and 6,401,854 a number of randomly distributed pixels in a solid-state x-ray imager, having non-destructive readout ability, are designated as reference pixels, to avoid missing exposures due to shading by the object being imaged. The distribution of reference pixels may be random or in a limited number of rows or columns. In one embodiment two sets of columns are selected as the reference pixels, preferably on opposite sides of the imaging array, and an average of the pixel values along a column is taken. The reference pixels are continuously monitored, and if a signal level of a predetermined minimum number of pixels exceeds a predetermined threshold, an image capture sequence starts. An image capture controller may also randomly select second reference pixels to determine an end of x-ray exposure. Additional circuitry is required on the array chip to select the reference pixels and compare their signal with a reference signal. The selected columns are reset at a 10 Hz frame rate to avoid dark current building up while waiting for an x-ray event and the rest of the array is held in reset to avoid any dark current building up. An end of exposure is determined either by a timer or by determining when signal on the reference signals is no longer increasing by more than an increase caused by the dark current.
U.S. Pat. No. 6,380,528 discloses a radiation sensor element integral with an array of image elements in which the radiation sensor output is preferably continuously supplying a signal and the radiation detector cell is preferably more responsive to incident radiation than an image element detector cell—for example the radiation detector cell is larger than the image element cell. The radiation detector cell may comprise a guard ring around the image array. Preferably threshold circuitry is coupled to the radiation sensor output and image acquisition is initiated responsive to a trigger signal from the threshold circuitry.
U.S. Pat. No. 6,775,351 discloses dental x-ray imaging equipment providing automatic detection of x-ray emission for transition to an image integration and acquisition phase. A solid-state imager, such as a CCD, is continually clocked during a standby phase prior to irradiation. A control unit analyses output of the imager, detecting variation caused by a start of x-ray emission using appropriate threshold levels. For example, pixels are identified for which an output exceeds a first threshold and the number of such pixels counted to determine whether the number exceeds a second threshold.
U.S. Pat. No. 6,797,960 discloses a radiation imaging assembly including bias monitoring means for determining radiation incident on an image element detector. The bias of either some or all of the detector elements is monitored, to provide a substantially instantaneous or real-time response to incident radiation by detecting increases and decreases in bias corresponding to increases and decreases in radiation intensity to initiate trigger signals. Changes in the bias current occur with changes in x-ray intensity incident on the imaging device due to charge pairs generated in a detection zone in response to incident radiation migrating to respective electrodes. The greater the intensity of x-radiation, the greater a number of charge pairs created.
In U.S. Pat. No. 6,856,350 a semiconductor radiation imaging device includes threshold circuitry configured to discard radiation hits on a pixel detector of the device which are outside a predetermined threshold range.
Therefore, principal shortcomings of the prior art may be summarised as follows. Some known imagers require additional off-chip sensing elements e.g. external photodiodes or fairs. These, and others which have additional on-chip trigger photodiodes, require separate readout circuits for the trigger signal. This additional complexity increases cost and reduces yield. The ESD/EMC susceptibility of the prior art devices also gives false triggers.
If image photosites are also used for triggering they cannot be separately optimized for both imaging and triggering.
Schemes which use a sparse array of image pixels for triggering require more complex address circuits or operational modes and risk introduction of fixed pattern noise in the image as a result of some pixels having been treated differently from others.
Some of the prior art is applicable only to CCD sensors, which add signals in the charge domain by means of special clocking. CCD-based trigger schemes tend to produce smeared or ghost images from x-rays received before the sensor is triggered.
It is an object of the present invention at least to ameliorate shortcomings in the aforesaid prior art.